Project Summary Breast cancer is the most commonly diagnosed cancer in women in the United States with a 1 in 8 lifetime risk for most women. It also ranks second highest in cancer related mortality. Nearly 75% of newly diagnosed breast cancer cases are estrogen receptor (ER) and often progesterone receptor (PR) positive and therefore treated using targeted endocrine therapies such as Tamoxifen (Tam) and Aromatase Inhibitors (AIs). Unfortunately 40% of patients with ER+ breast cancer will acquire resistance to these therapies and have a tumor recurrence. While the role of estrogen in breast cancer has been extensively studied, the role of PR has been largely overlooked. Activation of PR via progestins in hormone replacement therapy has been shown to be associated with an increased risk of developing breast cancer in post-menopausal women. It has been theorized that this may be due to expansion of the stem cell populations in the normal breast, which are expanded in response to progestins and are more susceptible to transformation. In the normal breast, these stem and progenitor cell populations expanded by progesterone are marked by expression of cytokeratin 5 (CK5). Similarly, the Sartorius laboratory found that in ER+/PR+ breast cancer cell lines, progesterone can expand the CK5 expressing cell population, which has been shown to mark a cancer stem cell (CSC) population in these ER+/PR+ cell lines. The lab also found that this CK5 expressing population is more tumorigenic both in vitro and in vivo, and more resistant to chemo- and endocrine therapies. A small molecule screen performed by the lab determined that select retinoids, including retinoic acid (RA), are able to prevent the induction of CK5 seen following P4 treatment of breast cancer cells. It is not known, however, how RA is able to prevent the induction of CK5 or whether the expression of CK5 is necessary to confer these cancer stem cell-like properties. The studies proposed here will unravel the mechanism by which hormones positively or negatively regulate a CSC population within ER+PR+ breast cancer, and have the potential to identify a means to target this CSC population with the ultimate goal of reducing recurrence and improving the outcome of this disease.